This invention relates generally to the laser art and more particularly to means for and a method of producing a phase conjugated data-carrying laser beam with predetermined temporal modulation.
It is well known that for any laser beam propagating through a medium there can exist a time-reversed replication of that wave. Thus, an appropriately patterned laser beam can travel through a light-scattering and/or distorting medium and merge as a laser beam having a uniform wave front, provided it is a replica, reversed in time, of a wave that was originally deformed by the same medium. Such a laser beam having a time reversed wave front is generally referred to as a phase conjugated laser beam.
It is also well known that phase conjugated laser beams can be generated by nonlinear effects in an irradiated medium. For example, nearly time-reversed laser beams have been obtained by stimulated Brillouin scattering of laser beams of high optical intensity. Phase conjugation has also been obtained using the photorefractive effect in selected materials.
Phase conjugation by four-wave mixing also been performed using single-domain crystalline barium titanate as the nonlinear mixing material. However, these procedures require external pumping of the barium titanate by counterpropagating laser beams in the same manner as earlier four-wave conjugators. Therefore, it is desirable in many applications to provide a compact and versatile apparatus for phase conjugation.
Prior art phase conjugation means may employ apparatus which implements the phase conjugation by four distinct processes: namely; stimulated Brillouin scattering, four-wave mixing, three-wave mixing, or photon echoes. Apparatus for implementing the phase conjugation by stimulated Brillouin scattering requires a nonlinear medium which is capable of undergoing stimulated Brillouin scattering. Three-wave mixing apparatus employs a second-order nonlinear medium and a laser oscillator operating at a predetermined frequency. The laser light interacts with the nonlinear medium and a second light beam to provide a phase-conjugated version of the second light beam. Four-wave mixing apparatus incorporates a third-order nonlinear medium, and at least one laser operating at a predetermined frequency. The laser light is imposed on the medium from at least two different directions and interacts with a third beam and the nonlinear medium. The third beam is then phase-conjugated. Photon echo apparatus essentially is comprised of a second laser operating at a predetermined frequency, and a nonlinear medium, the nonlinear interaction being substantially different from the nonlinear media of the other embodiments.
As will not be apparent, phase conjugation is the process by which a first laser beam is given a transverse phase profile identical to that of a second laser beam traveling in the direction opposite to that of the first laser beam. Transmission of laser beams a long distance through air or the like will result, for example, in significant beam spread because of turbulence, diffraction, thermal blooming and aerosol scattering. The laser beam will also suffer significant distortion.
The occurrence of such above noted effects makes data transmission by a laser beam difficult. However, as noted hereinabove, by pre-distorting the data carrying laser beam, it is possible to transmit this laser beam so that it arrives at least substantially unaberrated at its target or receiver with no beam spread and/or distortion. Pre-distortion or phase conjugation is useful in, for example, data links, satellite communication links, ground based laser beam transmission over free space, imaging lidar and the like.